Dan, Frances,
Wonderful link pointed out today. I did some quick checking against my shifters (embedded in Trimble postprocessing software and Blue Marble) and this is a very good site that exposes the 7 parameters with WKID's.
Frances, ITRF_2000_To_WGS_1984 is molendesky, 3 parameter, 0,0,0 bookkeeping transform. It merely is being used to Project, without projecting so another datum switch could be employed. Look at Pathfinder Office, coordinate system manager, datum tab, ITRF, and you'll see same shifters as expressed on Dan's link. So, explicitly, if you use WGS_1984_(ITRF00)_To_NAD_1983 + ITRF_2000_To_WGS_1984 in your case,
1) Excuse the order, any transform responds to your projection defined. So, your moving your NAD83 data, about 1.4 meters using WGS_1984_(ITRF00)_To_NAD_1983 which employs a 7-param. Helmert transform equivalent in Trimble as NAD 1983 (CORS96) and row 14 in the Excel spreadsheet that Dan Provided. Note, that just like Melita stated, ITRF2000 is equivalent to "WGS84" in the language of Trimble, the language of ESRI, and apparently the ITRF Transformation parameters.
2) Then, the second half of the composite shifter is really doing a "project define", by using 0,0,0 to "shift from WGS84 to your required destination definition of IRF_2000.
Datums are like going to the Dentist. Wait too long, and the pain of a visit increases over time. Brush often, and frequent check-ups (validation) is key to a healthy GIS (mouth).
Joel